The plant hormone abscisic acid [1] (FIG. 1) regulates many aspects of plant growth and development as well as responses to environmental stress (1). As used herein, brackets [ ] will be used to refer to chemical structures present in the attached FIGS. For example, in seed development, ABA induces synthesis of storage products, prevents germination of immature embryos and is involved in desiccation tolerance and germination of mature seed (1, 2). ABA levels in plants rise transiently in response to environmental stress and trigger a set of responses including rapid closure of the stomata reducing transpiration (1, 2). Numerous studies have been conducted to probe the structural requirements of ABA responses to develop analogs that are effective plant growth regulators (3, 4, 5). Some features of the ABA molecule [1], as shown in FIG. 1, appear to be required for activity, particularly the carboxyl and ketone groups, the six-member ring, the 7′-methyl group, and the cis double bond of the side chain. Other parts of the molecule can be modified without loss of activity. The ring double bond, both the 8′- and 9′-methyl groups, and the trans double bond of the side chain each can be altered and the resultant analog retains activity.
As shown in Scheme 1 (FIG. 1), ABA is catabolized predominantly through hydroxylation of the ring methyl groups or alternatively by conjugation to the glucose ester [7] (5, 6, 7, 8). The principal pathway of oxidation is through P450 monooxygenase mediated hydroxylation of the 8′-methyl group affording 8′-hydroxy ABA 2 which can rearrange to the closed form phaseic acid [3] (6). Alternative pathways, through hydroxylation of the 7′-methyl group affording 7′-hydroxy ABA [4] and the 9′-methyl group to give 9′-hydroxy ABA [5], which can also rearrange to the closed form neo-phaseic acid [6], have also been observed and contribute to ABA catabolism (7, 8). This catabolism by plant enzymes limits the practical application of ABA itself as a plant growth regulator (5). Metabolism resistant analogs of ABA altered at the 8′ carbon atom have proved to be more persistent and more active than ABA (5).